Nautical Chart
In this chart, the deepest depth on the chart is 20 feet while the shallowest depth is 2 feet. The black dots are the beacons. Beacons are like small floating lighthouses that flash a certain color of light. Sailors use them to determine where they are and which direction they should go. The green smiley faces are markers for the left side of the road and the red smiley face are markers for the right side. Together, they form a "road" in the sea which is in purple.
The nautical chart is essential for safe navigation. In conjunction with supplemental navigational aids, it is used by the mariner to lay out courses and navigate ships by the shortest and most economically safe route. Over 98 percent of the nation’s cargo is carried by waterborne transportation and all of those ships rely on nautical charts to find their way.